diff --git a/lib/rbs/errors.rb b/lib/rbs/errors.rb
index b07a792f6..c68f6814c 100644
--- a/lib/rbs/errors.rb
+++ b/lib/rbs/errors.rb
@@ -580,11 +580,13 @@ def initialize(message, location:)
   class TypeParamDefaultReferenceError < DefinitionError
     include DetailedMessageable
 
+    attr_reader :type_param
     attr_reader :location
 
     def initialize(type_param, location:)
       super "#{Location.to_string(location)}: the default of #{type_param.name} cannot include optional type parameter"
       @location = location
+      @type_param = type_param
     end
 
     def self.check!(type_params)
diff --git a/sig/errors.rbs b/sig/errors.rbs
index e83f51bf7..6f98fe4be 100644
--- a/sig/errors.rbs
+++ b/sig/errors.rbs
@@ -372,6 +372,8 @@ module RBS
   class TypeParamDefaultReferenceError < BaseError
     include DetailedMessageable
 
+    attr_reader type_param: AST::TypeParam
+
     def initialize: (AST::TypeParam, location: Location[untyped, untyped]?) -> void
 
     attr_reader location: Location[untyped, untyped]?